“I empathize with the desire to fight fire with fire, but Democrats should just never be in the business of taking unlimited corporate contributions,” Feingold told the audience of liberal activists and bloggers gathered here for the Netroots Nation convention, eliciting cheers. “It’s dancing with the devil, and it’s a game that we will never win.”

“Creating those kind of super PACs for Democrats is wrong,” Feingold said.

He added that the attitude among his fellow Democrats seems to be that they disagree with the Supreme Court’s Citizens United decision allowing for such groups to exist and influence elections, but that they would only change the rules after they win back control of Congress.

“Most people don’t change the rules after they win by them,” said Feingold, a co-sponsor of the 2002 law banning the use of soft money in campaigns. “We’ll lose anyway. We’ll lose our soul when it comes to the issue of corporate domination.

“I know people want to win. ... I like to win too,” Feingold added. “People will see us as weak and unprincipled. They will see us as corporate lite. Our strength is people power. We can raise that money from small contributions, and we can win without selling our soul.”

Feingold called on the activists attending the convention to back President Barack Obama’s effort to force government contractors to disclose their corporate donations. He said he hopes Obama is re-elected in 2012, adding that if the president gets a second term, the Democratic Party’s “top priority” must be passing the Disclose Act requiring new disclosure post-Citizens United.

The former Senator urged Obama to call for overturning Citizens United in “every speech, every statement” as he campaigns.

Marcy Wheeler of the "Firedoglake" blog introduced Feingold to the more than 1,000 in the auditorium at the Minneapolis Convention Center as “a guy who has been proved right again and again.”

Feingold was speaking at the gathering for the first time, even though he has long been a favorite of the crowd for his outspoken liberalism while serving in Congress.

If the liberals here were looking for clues about his political intentions, they probably left disappointed. The Democrat offered only a brief mention of his loss last fall to now-Sen. Ron Johnson (R). Feingold said nothing about the 2012 election to replace retiring Sen. Herb Kohl (D-Wis.), even though he polls best among the potential Democratic candidates.

He pointedly criticized Wisconsin Gov. Scott Walker (R), who could face a recall election in 2012. Polls show Democrats also would like to see Feingold run in that race.

Feingold did plug his new group, Progressives United, and offered a history lesson about union organizing and campaign finance.